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The most compelling argument against gay marriage is that it is a radical change to a timeless institution. “Every human society since the beginning of the human race has believed” that marriage is between a man and woman, says Roman Catholic Archbishop Salvatore Cordileone of San Francisco.

That sounds true. But it’s not. The story of marriage is far more complicated.

Dozens of cultures around the world practice what amounts to same-sex unions. Many Native American tribes believe people can be born with two spirits — male and female. Unions between “two-spirited” people and members of their own sex were permitted. In parts of Africa, a childless woman can marry a single mother in a traditional ceremony, sharing in the financial burden — and benefits — of her partners’ children. Those “woman marriages” aren’t about sex but rather the necessity of offspring. It gives us a glimpse of how radically the notion of marriage can differ across cultures.

Even heterosexual marriage has evolved over time.

For most of human history, it was not about “one man and one woman” but rather one man and as many women as he could afford. Eighty-five percent of human societies are polygamous or have been in the relatively recent past, according to Joe Henrich, co-director of the Centre for Human Evolution, Cognition and Culture at the University of British Columbia.

Thousands of years ago, death rates were high. Many children had to be born to assure family survival. Parents married off their young daughters to older men who paid hefty dowries in exchange. Well-off men married as many wives as they could; impoverished men didn’t marry at all.

Only 15 percent of human societies are considered “monogamous” by anthropological records, including some that eked out a living in environments so harsh that no man could possibly provide for more than one wife.

“What surprised me the most,” Henrich said, “was how rare monogamy was in a global sense, and in a historical sense.”

In fact, it was so rare that it presented a mystery: Why did the notion of one-man-one-woman marriage take root in Europe? Why did it spread?

Henrich and two colleagues sifted through ethnographies, demographic trends and crime rates. They came up with a surprising conclusion: Having a large pool of unmarried guys hanging around is just too chaotic for society. Wifeless men get frustrated and jealous. They turn to rape, robbery and kidnapping. Rebellions break out. Leaders adopted monogamy to placate these reckless forces and spread wives around more fairly.

Some cultures, like ancient Egypt and Babylonia, imposed monogamy on the poor but made an exemption for the aristocracy. Others adopted it wholesale — and prospered.

The Roman Empire enforced monogamy on its subjects and enjoyed hundreds of years of unprecedented stability. Christians took power in the fourth century and followed suit. By the 10th century, polygamy had all but disappeared in Europe. In the 11th century, Judaism formally gave it up.

The newfangled idea of monogamy ushered in other changes: Women married later and had fewer children. Marriage became less about wealth and clan survival and more about love and personal choice.

Around the 15th century, Henrich said, “We begin to get the appearance of the modern marriage pattern.”

It’s no coincidence Shakespeare wrote “Romeo and Juliet” in the late 1500s, about lovers torn between passion and the duty to marry for your family.

“Today, we all root for Romeo and Juliet,” said Bruce Frier, a professor of classics and Roman law at University of Michigan. “But at the time, it would have been more contentious.”

Today, the notion that love should trump all else in marriage is widespread. That was the real revolution, and it has already taken place, at least in the Western world. Take that view, Frier said, and the debate over gay marriage becomes a tiny blip in a much larger trend.

So where does Henrich’s research take us from here? Conservatives might celebrate his conclusion that Europe adopted “traditional marriage” because it turned out to be a better, more fair, more peaceful system. But Henrich’s research isn’t an indictment of gay marriage.

By his logic, any marriage system that lets a large number of unmarried men finally get hitched would lead to greater stability. Legalizing gay marriage certainly does that.